1 / 15

Initial Work on Suspension Diagonalisation – August 2004

This work focuses on the diagonalisation of suspensions to minimize pitch and yaw motion by adjusting the amplitude of signals going to each coil. Step-by-step instructions are provided for diagonalisation at low and high frequencies. Filter shapes, matrices, and transfer functions are discussed for optimal performance. Suggestions for improvement and further work are also highlighted.

jeanieb
Download Presentation

Initial Work on Suspension Diagonalisation – August 2004

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Initial Work on Suspension Diagonalisation – August 2004 Bryan W Barr

  2. Basic Concept… UR UL LR LL Force is applied to the mass by applying a signal to the 4 coils (UL, UR, LL and LR). If the force is not the same from all 4 coils then the mass will not be pushed cleanly in position, but will have some element of pitch and yaw motion. The aim of diagonalisation is to adjust the amplitude of the signals going to each coil such that the total force applied has minimal pitch and yaw coupling.

  3. Step by Step • Take transfer function of pos->pitch and pos->yaw as a reference • Diagonalise at low frequency: ~DC (note coil gains) • Diagonalise at high frequency: 25 Hz (note coil gains) • Work out the HF/DC ratio for all four coils • Generate a filter to convert HF (gain=1) to DC (gain=ratio) and apply them to the Pos Output Filter • Take transfer function of pos->pitch and pos->yaw and compare with reference – it should be better

  4. Diagonalise at DC • Essentially, I went the simple route (possibly not the best though) of applying a DC offset of a few thousand counts to the LSC path of an optic • Concentrating on the BS optic since that’s the one that seemed to have the highest priority • Before applying the offset – note the values of the OSEM PITCH and YAW (oplevs change more but aren’t available on all optics) • Apply offset and change the coil gains stepwise (and symmetrically) until the PITCH and YAW values are back to the start position • Check by changing the offset – if the values change then it’s not diagonalised

  5. Diagonalise at HF • This time, a sine wave is applied to the LSC_EXC point of the optic (at 25Hz) – I found 10000 counts to be OK-ish • Take a power spectrum of the signal around the sine wave frequency – at least 10 averages and using a BW of 0.1Hz – can probably play around with this though • Note the values of the peaks produced and adjust the coil gains to reduce the height of the PITCH and YAW peaks

  6. HF diagonalised • No “before” picture here. Only after. • Here : Pos=156, Pit=3.5, Yaw=1.1 are the optimised diagonalisation peaks at 25 Hz

  7. Matrices • DC matrix: 1.004 1.019 0.792 1.005 • HF matrix: 1.014 1.014 0.986 0.986 • Relative gains: 1.010 0.995 1.014 0.981

  8. Filter Shape • ULPOS DC->HF filter • Note the gain difference between low and high freq.

  9. Transfer functions Reference plot After filters Note the poor YAW performance. Thought alignment may have drifted so reset DC alignment and tried again.

  10. After DC Matrix Reset Reference plot After filters YAW is much better but PITCH is worse than for previous matrix and both are worse at high frequency. Hmmn… have I done something daft? The answer to that is a resounding yes!

  11. Ooops! Had the Silly Thing in Reverse! • Rather embarrassingly, it appears I can’t count properly – in order to convert to DC from HF one requires gains of DC/HF instead of HF/DC • I quickly threw together some filters and did a transfer function…

  12. New Improved Formulae Reference plot With Proper Filters

  13. Status… • Not the best we can get? Probably not – the question is how do we get better… • Possibly change the filter resonant frequency to adjust the coupling at the pendulum peak – may not be correct • Is there a better way of diagonalising? • Currently, only the BS is done • Could quickly diagonalise at DC for the other suspensions in the interim but the HF part takes longer…

  14. The End • Needs more work • Comments, suggestions and insults all welcome. • Back in 6 weeks

  15. Details… • The files included here can be found in caltech/dvsave/bryan/ • Also, the small bit of scripting which I used to diagonalise at DC is in the scripts/test/ directory

More Related